I'm assuming you are talking about a UPS which has an inverter to convert the 12 volts DC from the battery to 120 VAC.
PS, use a CFL, and a 30 watt bulb will give you the same amount of light.
UPSs come in many different sizes and storage levels. I don't think they come as small as 100 watts. But that's good, as they all have limited storage capability.
For example, here is a 330 watt unit
http://www.amazon.com/APC-BE550R-Back-UPS-8-Outlet-Battery/dp/tech-data/B000KBG51W/ref=de_a_smtd
that states "Up To 44 Min Battery Backup Time" but doesn't state at what power that 44 minute number is measured at.
here is another 330 watt unit
http://www.amazon.com/APC-Back-UPS-Outlet-550VA-120V/dp/tech-data/B0019804U8/ref=de_a_smtd
that lists "Full Load Run Time: 3.3 minutes"
which is way too short for your application, even tho at 100 watts you will get 19 minutes.
I's suggest you continue to search the internet. Search for "UPS" and look for the power rating in watts, and run time specs.
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